Like the others said, fill out the sticky.
As for top speed, that is a terrible thing to be concerned about! Just about any bike will get you up to a speed that can do you serious harm. How quick it accelerates is a more important factor. How well the suspension works is critical, as that is what is going to save you when you hit something going too fast.
Also keep in mind that there will be some inherent differences in how a quad handles and how a bike handles. Quads are great in sand, bikes tend to suck. If you ride sand dunes a lot you may not be happy on a bike. Quads can crawl up/down a hill slowly. A bike has to maintain a minimum speed to be stable.
Where bikes excel over a quad is when the trail gets a little rough. With a bike you only need to find a 4" wide path through the rocks. With a quad you need two 10 inch wide paths the exact right distance apart. Bikes weigh less and have a lot more suspension travel.
So fill out the sticky (at the top of this forum topic) and we will give you more specific recommendations.
Rod